1. Field of the Invention
This invention relates to unexpanded, unsintered, and extruded films of polytetrafluoroethylene (“PTFE”), and to the process for producing such films.
2. Description of the Prior Art
Conventionally, unexpanded and unsintered PTFE films are prepared by a process that typically begins by mixing powdered PTFE resin with a solvent to produce a paste. The paste is preformed to remove air, extruded into a film, and then calendered to a desired thickness. The calendered film is then dried to evaporate the solvent.
Films prepared in this manner have specific gravities ranging from about 1.5 to 1.7, thicknesses ranging from about 0.002″ to 0.010″, and tensile strengths in the machine direction ranging from about 700 to 1800 psi, depending on film thicknesses. Such films conventionally serve many products. One is in the service of components of multi-layered composite products, a prime example being the flexible high strength laminate marketed by Textiles Coated International of Amherst, N.H. under the tradename “LFP,” and described in U.S. Pat. No. 5,466,531.
There is now developing a demand for significantly thinner multi-layer products incorporating unexpanded PTFE film components. The majority of the conventionally produced PTFE films have thicknesses that rule them out as candidates for these thinner products. Those films that are thin enough to qualify are either too porous to provide the barrier properties typically required of multi-layer composites, or they are produced by processes that contribute prohibitively to their costs.